Field of the Invention
The present invention generally relates to heat spreading lids for electronic modules. In particular, the invention relates to reducing TIM (thermal interface material) strain in electronic modules through lid designs.
Description of the Related Art
TIM (thermal interface material) is used in electronic modules to allow for heat transport from an IC (integrated circuit), referred to as “a die”, to a heat spreading component, such as a lid or cooling plate, in order to keep the IC within a specific operating temperature. The TIM material is thermally cured to both the IC and the heat spreading component providing both thermal and mechanical coupling. TIM strain in electronic modules occurs from the CPI (chip package interaction) due to the thermal mechanical coupling resulting from the differential thermal coefficients of integral component (die, controlled collapse chip connection (c4 interconnects), under fill, and organic carrier). CPI occurs during thermal expansion and/or contraction, such as lid assembly curing, accelerated reliability cycling and power on off cycling during operation. For example, the die may have a coefficient of expansion of three parts per million, and the organic carrier or circuit card may have a higher coefficient of expansion of seventeen to twenty parts per million. This can lead to the die crowning at certain temperatures where the die will attempt to pull away from the lid and in the process strain the TIM. This can result, for example, in separation of, or cracks in, the TIM at the corners of the die, which can in turn lead to TIM failure. When a TIM failure occurs, it may reduce heat dissipation for the electronic module, which can result in over heating and potential failure of the die.
An increase in lid thickness, which is desirable for improved heat spreading, increases the lid stiffness which lowers lid-to-die compliance and increases TIM strain, leading to a trade off in thermal performance versus TIM reliability. Also, as die areas increase, TIM strain also increases. One technique to mitigate the TIM strain is to minimize the CPI by increasing the thickness and stiffness of the organic carrier core. However, this is a costly adder to the module's overall cost. Newer generations of electronic modules are attempting to reduce costs by eliminating the organic carrier core resulting in increasing CPI and TIM strain. Therefore, innovative solutions to reduce TIM strain while improving thermal performance are needed.